So my daughters betta has been through a lot. I switched him to a larger aquarium and he loved it until he got Cotton mouth. I cured him of that and he was fine, now he sits under the filter at a 90 degree angle and wont swim around like he use too! He has been doing this for a couple of weeks. My husband bought an all-in-one treatment but that didn't help. Please help my daughters fish

What size tank is he in?
How often do you do water changes?
What temp is the water?
Do you use water conditioner or distilled water?
What are the water parameters?
We can help you alot better if you answer those questions. Pics are helpful too